使用Field和literal String在几分钟内完成Datediff

I am having issues trying to calculate the difference in Minutes between a Field and a literal string. However the output I am getting doesnt seem right at all, it looks to me like you can't seem to compare a Field and a literal string, however if they both the same then it seems fine i.e both are column values or literal strings.I am having issues trying to calculate the diff